Směsné oxidy s obsahem kovových promotorů pro katalytický rozklad N O

Abstract

The catalytic decomposition of N2O over Co-Mn-Al mixed oxide prepared by calcination of appropriate synthetic hydrotalcite or over promoter (Li, Na, K, Rb, Cs, Pd, Pt, La, Ce) modified Co-Mn-Al mixed oxides prepared by (i) coprecipitation and (ii) impregnation of Co-Mn-Al mixed oxide. The catalysts were characterized by physical adsorption of nitrogen, AAS, AES, XRD, IR, SEM-EDX, TPR and TPD. N2O decomposition was performed in the fix bed reactor in temperature range 300 – 450 °C, at space velocity 20 – 60 l g-1 h-1. Inlet mixture contained according to the type of experiment 0.1 mol.% N2O, 0 – 5 mol. % O2, 0 – 0.17 mol.% NO, 0 – 0.17 mol.% NO2 and 0 – 4 mol.% H2O balanced by He. The physico-chemical characteristics of the catalysts and catalytic activity in inert atmosphere and simulated real conditions were correlated. The specific surface area of prepared catalysts was 60 – 111 m2 g-1. Spinel phase was identified by X-ray diffraction in all catalysts. Reducibility of Co-Mn-Al mixed oxide was changed with type and promoter amount. Homogenous promoters distribution on catalysts surface was verified by SEM-EDX analysis. Rare earth metals (La, Ce) or noble metals (Pd, Pt) promoted catalysts had the same or lower catalytic activity in comparison with non-modified Co-Mn-Al mixed oxide. The alkali metals were the suitable promoters in Co-Mn-Al mixed oxide. Catalytic activity for N2O decomposition decreased in order: Cs < Rb < K < Na ≥ non-modified mixed oxide < Li. The Co-Mn-Al mixed oxide catalysts with the same molar alkali content were tested for N2O decomposition under real condition corresponding to waste gas composition from nitric acid plant. Co-Mn-Al mixed oxide promoted with 4.1 wt.% Cs was the most active catalyst in presence of O2, NOx a H2O. The lower temperature of oxygen desorption and higher amount of desorbed O2 was observed by TPD-O2 and TPD-N2O from K-promoted Co-Mn-Al mixed oxides which were the most active for N2O decomposition. From transient responses experiments the higher initial rate of oxygen formation was observed. The lower temperature of oxygen desorption was, the higher oxygen formation rate was. Higher desorbed NO amount in TPD-NO was observed from K-promoted catalysts active for N2O decomposition in presence of NO. The cationic redox mechanism was assumed for N2O decomposition over Co-Mn-Al mixed oxide catalysts. That was confirmed by correlation between catalysts work function and catalytic activity. The more active the catalyst was, the lower value of work function was observed. N2O chemisorption on the catalyst surface was the first step of N2O decomposition. Oxygen desorption was the rate determining step. O2 desorption by recombination of adsorbed oxygen atoms by Langmuir-Hinshelwood mechanism was prevailling. Nevertheless, O2 desorption by Eley-Rideal mechanism was not excluded. The presented results will be used for pilot-plant catalysts preparation for N2O decomposition in waste gas from nitric acid plant.